When i worked for a pool company in hs/college, blowing out the lines meant sending compressed air through the lines to blow out any garbage in them.
A pressure test involved plugging all of the lined in the pool and then blowing in compressed air measured by a pressure guage. Plugged lines would hold the air pressure like a baloon. If there was a leak, you would see the air pressure steadily drop . . .
With your description of the flow starting off strong and then dropping over the next few days, it certainly sound like a "leak." However, you won't see a lot of water loss because the leak is in one of the incoming lines, either the skimmer or the main drain (assuming you have one).
Do you leave your pump and filter outdoors for the winter? Another common problem around here is that the pump basket housing cracks due to ice expaning. It's a small hairline crack, so it's not visible, but it will suck in air.
I feel for you, cuz isolating the problem can be a real biotch . . .